Minus Filters or Notch Filters are by definition filters that eliminate a wavelength band from a spectrum, by reflection or absorption. The wavelengths that are eliminated are called the rejection band. The desired transmittance outside of the rejection band should be high. Conventional dielectric interference of the above type are generally comprised of a plane surface substrate, such as glass or germanium, upon which has been deposited by e.g. vacuum deposition a plurality of alternate high and low refractive index thin uniform layers where each layer has a quarterwave optical thickness of an odd multiple of a quarterwave, at the chosen wavelength which is required to be reflected. This multilayer system is known as a periodic multilayer system. Such systems exhibit a reflection zone centered at .lambda.o, which is the design wavelength where the layers have an optical thickness of a quarter-wave.
The width of the rejection zone depends on the ratio between the refractive indices of the high and low index layers. The number of periods control the amount of rejection and the steepness of the edges of the zone. Obtaining a narrow rejection zone dictates the use of a multilayer consisting of two materials with a relatively small difference between their indices of refraction. There is an immense difficulty in finding such materials that are convenient and practical, especially in the infra-red region, where only a limited number of materials are available.
However, there are several methods to produce these layers with small index difference. One method proposes using a material with an index of refraction that varies sinusoidally with the deposition parameters. When the variations are continuous the filter is called a Rugate Filter. Another method is the use of codeposition of the materials with gradual changes in concentration of the mixture resulting in gradient index. This technique was recently used to design and fabricate a single line and multiple line Rugate filters.
Another method uses Herpin Equivalent Layers, where one layer with a nonexisting index of refraction is fabricated from at least three real layers, to substitute one index with an index close to that of the other material. This method was applied in U.S. Pat. No.4,832,448. When applied for use in the visible region, it results in relatively much thinner layers than regularly used and requires the use of many periods and needs high accuracy of the layer monitoring.
It is the object of the present invention to provide a narrow-band notch filter in which these disadvantages are avoided.